You didn't specify what kind of wildlife, but with some there is one absolute rule: Whatever focal length you have, you need a bit longer.
For that reason, I don't think you'll get enough with the 100-400 coming from your 280mm (200+1.4x). But if you think 400mm is enough, rather than buy the 100-400, I think you might do as well to get the Canon 2xII extender. That would give you a 400mm F5.6--same as the 100-400 at 400mm, but at a fraction of the cost.
Otherwise, if you want more reach for not too much cost, look at the Sigma 50-500 or their new 150-500OS.
FYI, so you can measure the motives of the the person recommending, I have the 100-400 and also the Sigma 50-500. Also the Canon 500mm F4L, which I often use with the 1.4xII extender. I really like the 100-400 under a lot of circumstances, especially as a handheld. HOWEVER, if I already owned a 70-200 F2.8, I would get the 2xII extender.